Container for radioactive inventory and method of making same

ABSTRACT

A container for radioactive inventory has an end wall, a side wall, and a container lid that form a closed chamber for the radioactive inventory. A plastically deformable layer is provided between the container lid and the inventory and is of such a composition that, in the event of an impact of the inventory against the container lid, at least a majority of the impact forces are uniformly distributable over at least a majority of a surface of the plastically deformable layer.

FIELD OF THE INVENTION

The present invention relates to a container for radioactive inventory.More particularly this invention concerns such a container and a methodof making it.

BACKGROUND OF THE INVENTION

A standard container for nuclear inventory has an end wall, a side wall,and a lid. The container forms a closed chamber for the radioactiveinventory.

In such containers the connection of the container lid to the side wallforms under extreme circumstances a relative weak point because theconnection is often reversible. For example, screws are understood asreversible connections. Since such the containers are characterized by avery long life, the containers have to be configured for a number ofextreme situations. Herein included are, for example, extreme impactsituations such as falls from a height of several meters. In the case ofsuch falls, the potential energy of the container is converted intodeformation energy, and, based on the solid construction of thecontainer walls, the greatest force is exerted on the screws, inparticular when the container lands in a so-called “lid flat fall” withthe lid down, that is on its lid. If the yield point of the screws isexceeded, the screws are plastically deformed. In an extreme case, theplastic deformation can lead to a leakage of the container.

From practice is it further known to provide containers holding fuelelements with plastically deformable shock absorbers within thecontainer. These shock absorbers are cylindrically tubular and made ofaluminum. In the case of a fall through several meters on the horizontallid, the fuel elements crash onto the hollow cylindrical shockabsorbers, as a result of which the shock absorbers are plasticallydeformed in an accordion-like manner. This plastic deformation of theshock absorbers dissipates a considerable portion of the potentialenergy so that the stresses in the screws are reduced at least to theextent that no plastic deformations of the screws results. However, thehollow cylindrical shock absorbers from the prior art known from thepractice are attached via further screws to the inner face of the lid.Consequently, threaded holes are required on the inner face of the lid,as a result of which the lid loses stability. Moreover, the manufactureof the hollow cylindrical shock absorbers as well as their attachment onthe inner face of the lid require a certain effort. After all, theinventory in form of the fuel elements has to be guided to the shockabsorbers by guide heads to ensure that the inventory does not slidepast the shock absorbers and, in this way, crash in an uncushionedmanner onto the lid.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide animproved container for radioactive inventory and method of making same.

Another object is the provision of such an improved container forradioactive inventory and method of making same that overcomes theabove-given disadvantages, in particular that avoids the disadvantagesdescribed above, in particularly by being of simpler construction withrespect to the manufacture and installation of the shock absorbers.

SUMMARY OF THE INVENTION

A container for radioactive inventory has according to the invention anend wall, a side wall, and a container lid that form a closed chamberfor the radioactive inventory. A plastically deformable layer betweenthe container lid and the inventory is plastically deformable such that,in the event of an impact of the inventory against the container lid, atleast a majority of the impact forces are uniformly distributable overat least a majority of a surface of the plastically deformable layer.

For example, pellets, sheared steel scrap and solid elements areconsidered as radioactive inventory. The radioactive inventory may alsobe at least one barrel or a plurality of barrels. In addition to theradioactive inventory, water can also be in the container. For thisreason, the container for radioactive inventory is suited to isolateinventory from the surroundings in a fluid-proof manner. Moreover, thecontainer is suited for a sufficient radioactive shielding via thickmetal-like container walls. The end wall and/or the lid is/arereversibly or irreversibly connected to the side wall. Screws, forexample, represent reversible connections. Welded connections, forexample, represent irreversible connections. Preferably, the end wall isirreversibly connected to the side wall. It is advantageous that the lidis reversibly connected to the side wall.

Most of all solid bodies are, to a certain degree, elastically and alsoplastically deformable. The expression “plastically deformable” in termsof the present invention, for this reason, means that the body isprimarily plastically deformable. In particular hollow structures ofmost different kinds made from most different materials are includedtherein. The empty chambers can have regular, geometric shapes such ashoneycombs. The empty chambers can, however, also be designed in asbubbles so that the plastically deformable element then is a solidifiedfoam.

The term “layer” can reference a completely flat element but also aplurality of elements distributed over an area. For this reason, theplastically deformable layer can assume different shapes. Theplastically deformable layer can, for example, be a circle or a squareor comprise a plurality of rings concentric to one another. The layercan be a checkerboard or a plurality of points. A combination ofdifferent area shapes is also possible.

In order of being able to uniformly distribute the impact forces, thelayer has to be designed in a continuous manner or the individualelements of the layer have to be designed approximately equal instrength. This ensures that the forces of impact of the inventory is nottransferred too strongly at some points to one or a plurality ofprotruding layer regions. In particular, solid bodies are not allowed tobe between the plastically deformable layer regions or surrounding thelayer regions in such a manner that the impact forces are transferred intotal or in part to the lid and, in this way, that the plasticallydeformable layer is bypassed. In particular, the plastically deformablelayer is configured such that, for example, a plate is placed onto theplastically deformable layer so that it can uniformly distribute theimpact forces of at least a majority of the inventory to at least amajority of the surface. The term “majority” means preferably 50%, morepreferably 75% and particularly preferably 100%. Such a configuration ofthe plastically deformable layer ensures that the kinetic energy of theimpact can be distributed over a particularly large area, as a result ofwhich the plastically deformable layer can be designed considerablythinner.

It is within the scope of the present invention that the plasticallydeformable layer has a specific energy absorption of 5 to 50 J/cm³,preferably of 15 to 40 and, particularly preferably, of 20 to 30 J/cm³in the vertical direction. The specific energy absorption is asubstantially volume-independent measurement of the plasticallydeformable layer. The lower the specific energy absorption, the thickerthe plastically deformable layer has to be so that a respectively highamount of kinetic energy can be absorbed by the plastically deformablelayer. However, the specific energy absorption cannot be arbitrarilyhigh because otherwise the stresses in the screws can become so greatthat plastic deformations also occur in the screws. The indicated rangesof the specific energy absorption are largely independent from theconfiguration of the container and of the inventory so that theyrepresent a basic statement about the nature of the plasticallydeformable layer. The specific energy absorption can particularly beadjusted via average empty chamber volumes or via the respectivematerial. The higher the empty chamber volumes, the harder the materialsystem for a specified, desired specific energy absorption has to be,and vice versa.

It is within the scope of the present invention that the plasticallydeformable layer is isotropic. In this instance, the term “isotropic”means that the plastically deformable layer is plastically deformable inan approximately equal manner in all spatial directions. An example forisotropic plastically deformable materials is solidified foams. Incontrast, empty chamber structures having geometrically regulardimensions such as honeycomb chambers are, as a rule, not isotropicallydeformable.

Preferably, the plastically deformable layer is a metal foam. The metalfoam advantageously has closed cells. It is within the scope of thepresent invention that the plastically deformable layer is aluminum and,according to a particularly preferable embodiment, is made out ofaluminum foam or substantially out of aluminum foam. Advantageously, thealuminum foam is made out of at least 90 wt %, preferably out of atleast 95 wt % and particularly preferably out of 97 wt % aluminum.Preferably, the plastically deformable layer is an Al—Mg—Si alloy. Morepreferably, the plastically deformable layer has remnants of a foamingagent. Very preferably, the plastically deformable layer has remnants oftitanium.

It is preferred that the plastically deformable layer is encapsulatedand/or coated. Preferably, the plastically deformable layer isencapsulated and closed cell. The encapsulation ensures that, in thecase of a closed cell metal foam, micro cracks are counteracted and, inthis way, the plastically deformable layer is kept water-tight. Thewater-tightness prevents oxidation of the metal foam, as a result ofwhich the metal foam has consistently good deformation properties over along period of time.

It is within the scope of the present invention that the plasticallydeformable layer has a thickness of 30 to 200 mm, preferably 40 to 150mm and particularly preferably 50 to 100 mm. The density of theplastically deformable layer is preferably 0.1 to 2 g/cm³, furthermorepreferably 0.2 to 1.3 g/cm³ and particularly preferably 0.5 to 0.9g/cm³. Preferably the plastically deformable layer is configured in acircular and/or annular manner.

Preferably, the plastically deformable layer is attached directly to theinner face of the container lid. The term “directly” means that, inparticular, nothing encloses the plastically deformable layer or isbetween the plastically deformable layer and the lid.

According to one embodiment, the plastically deformable layer isunitarily bonded to the lid. Preferably, the plastically deformablelayer is adhesively bonded or welded and, particularly preferably, theplastically deformable layer has, during foaming, been molded to thelid. According to a further embodiment, the plastically deformable layeris attached to the lid by screws.

Another preferred embodiment of the present invention is characterizedby the fact that a lead shielding lid is interposed between the lid andthe plastically deformable layer. In this instance, it is within thescope of the present invention that the lead shielding lid is attachedto the container lid by a flat holding element, in particular, by aplate. In this case, the flat holding element or plate is between thelead shielding lid and the plastically deformable layer and, for thisreason, the assembly of the lead shielding lid and the flat holdingelement or holding plate is positioned between the lid and theplastically deformable layer. In this embodiment, the plasticallydeformable layer is advantageously attached directly to the leadshielding lid or to the flat holding element or to the holding plate. Inthis instance, the plastically deformable layer can be attached byscrews. Alternatively or additionally, the plastically deformable layercan also be unitarily connected to the lead shielding lid or to the flatholding element. In this way, the plastically deformable layer can beadhesively bonded or welded onto the lead shielding lid or onto the flatholding element and, according to one embodiment, the plasticallydeformable layer can, during foaming, be molded to the lead shieldinglid or to the flat holding element. According to a recommendedembodiment, attaching the plastically deformable layer “directly” to thelead shielding lid or the flat holding element means in particular thatnothing encloses the plastically deformable layer and that nothing comesbetween the plastically deformable layer and the lead shielding lid orthe flat holding element.

In general, it is also within the scope of the present invention thatthe plastically deformable layer is enclosed by a casing, in particular,by a water-tight casing. Furthermore, it is also within the scope of thepresent invention that the plastically deformable layer has been foamedin an envelope, in particular, in a water-tight envelope.

According to a recommended embodiment of the present invention, a leadshield is also provided on the container inner face of the side wall andof the end wall. Then, it is within the scope of the present inventionthat the total interior of the container is encapsulated by a leadshield. The thickness of the lead shield is preferably between 20 and 40mm.

Advantageously, the side walls of the plastically deformable layerenclose a fluid at least in areas. The term “fluid” refers to liquid andgas and, in particular, air or water. Advantageously, the side walls ofthe plastically deformable layer are spaced from the inner face of theside wall or from the inner face of the lead shield at the side wall.Preferably, the spacing between at least one side wall of theplastically deformable layer and the inner face of the side wall or theinner face of the lead shied is 0 to 100 mm, in particular 10 to 90 mmand preferably 20 to 90 mm.

According to a preferred embodiment, a load distributor is between theplastically deformable layer and the inventory. Advantageously, the loaddistributor is a load distribution plate. According to anotherembodiment, the load distributor is a basket lid enclosing theradioactive inventory. According to another embodiment, the radioactiveinventory itself is provided with a surface facing the lid and parallelto the lid. It is within the scope of the present invention that thesurface of the radioactive inventory facing the lid and parallel to thelid is formed by small parts of elements, for example, pellets.

According to a particularly preferable embodiment, the load distributorcomprises a load distribution plate. Advantageously, the loaddistribution place is made out of fine-grain construction steel. It ispreferred that the thickness of the load distribution plate is 5 to 40mm, furthermore preferably 10 to 30 mm and particularly preferably 15 to25 mm. The 0.2% yield point of the load distribution plate isadvantageously 600 to 1600 MPa, furthermore advantageously 800 to 1400MPa and particularly advantageously 1000 to 1200 MPa. These measuresprevent in particular a punching through of the load distributor.

Advantageously, the lid is attached to the side wall via reversiblefasteners. Preferably, the reversible fasteners are screws. The screwsof the screw connections have external threads of advantageously 24 to64 mm, preferably 30 to 56 mm and particularly preferably of 36 to 48mm.

It is within the scope of the present invention that the side wall has athickness of 100 to 350 mm, preferably of 120 to 250 mm and particularlypreferably of 140 to 180 mm. The interior height of the container isadvantageously from 0.5 to 10 m and preferably from 0.5 to 5 m.According to a particularly preferred embodiment of the presentinvention, the interior height of the container is from 0.6 to 2 m,particularly from 0.7 to 1.5 m. Preferably, the side and end walls arecast as one piece. It is preferred that the side wall and the end walland also the lid are of cast iron. The cast iron is preferablyattributed to Quality GGG 40.

In order to solve this problem, the present invention further teaches amethod of making a container for radioactive inventory, in particular ofmaking a container according to the present invention, the containerhaving an end wall, a side wall, and a lid and enclosing an interiorspace for the radioactive inventory, at least one plastically deformablelayer being between the container lid and the inventory, and theplastically deformable layer being configured in such a manner that theimpact forces of at least a majority of the inventory are uniformlydistributable over at least a majority of the surface of the plasticallydeformable layer. As described above, the container, according to aparticularly preferable embodiment of the present invention can alsohave a lead shield on its inner face, the lead shield advantageously onthe inner face of the lid and/or on the inner face of the side walland/or on the inner face of the end wall.

It is within the scope of the present invention that the plasticallydeformable layer is made out of a metal foam or substantially is madeout of a metal foam. According to a particularly recommended embodiment,the plastically deformable layer is made out of an aluminum foam or issubstantially made out of an aluminum foam. Preferably, the metal foamis foamed by a foaming agent. Preferably, the foaming agent is titaniumhydride. According to a particularly preferable embodiment, the metalfoam is unitarily bonded by foaming to the lid or to the lead shieldinglid or to the flat holding element on the lead shielding lid.Preferably, the foaming also unitarily connects the metal foam to a loaddistributor. Categorically, attaching or connecting the plasticallydeformable layer or the metal foam can also occur via screw connections.

The discoveries of the present invention are that the plasticallydeformable layer results in considerably simplifying the container. Afurther consequence is that the shock absorber in the form of theplastically deformable layer has a smaller height, as a result of whichmore usable space is available. In particular, employing the metal foamenables economic manufacture of the shock absorber that almost entirelycan absorb the kinetic energy. By encapsulating or coating, the metalfoams can be designed having a particularly long service life. Loaddistributors enable a surface distribution of the impact forces andprevent punching through the plastically deformable layer. As a result,the plastically deformable layer can be designed in an even thinnermanner without risk.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become morereadily apparent from the following description, it being understoodthat any feature described with reference to one embodiment of theinvention can be used where possible with any other embodiment and thatreference numerals or letters not specifically mentioned with referenceto one figure but identical to those of another refer to structure thatis functionally if not structurally identical. In the accompanyingdrawing:

FIG. 1 is a schematic cross-section of the container according to thepresent invention;

FIG. 2 is a schematic view of another embodiment of the containeraccording to FIG. 1;

FIG. 3 is a schematic view of yet another embodiment of the containeraccording to FIG. 1.

SPECIFIC DESCRIPTION OF THE INVENTION

FIG. 1 shows a container according to the present invention having anend wall 2, a tubularly cylindrical side wall 3 centered on a normallyvertical axis A, and a lid 4 here shown closing the open bottom end ofthe side wall 2. The container is hollow and cylindrical and forms aclosed chamber 5 in which an only schematically represented radioactiveinventory 1 is located. The lid 4 is connected via screw connections 8in the form of 24 M36 screws to the side wall 3. The container walls 2,3, and 4 are made out of cast iron of quality GGG 40. The side wall 3has a thickness of 160 mm; in contrast, the end wall 2 and the lid 4each have a thickness of 180 mm. The hollow cylindrical interior space 5has a height of 1140 mm and a diameter of 740 mm. The side wall 3 and 3end wall are unitarily cast with each other.

The container is shown upside down to illustrate a lid flat fall. At thetime of lid-first impact onto a solid base, the inventory 1 strikes thelid 4 with a time delay so that significant forces act upon the lid 4.

According to the invention a plastically deformable layer 6 is attachedto the inner face of the lid 4 in the form of a block of aluminum foamhere formed as a cylindrical plinth of a diameter smaller than theinside diameter of the side wall 3. The aluminum foam has been foamed byuse of the foaming agent titanium hydride. The thermal energy expendedduring foaming results in a liquefying of the aluminum and, when thealuminum foam cools, it bonds to the lid 4. During foaming, the aluminumfoam of the deformable plastic layer 6 is limited in the upwarddirection by a load distributor 7 in the form of a load distributionplate of cylindrical shape and a diameter equal substantially to theinside diameter of the side wall 3. The load distributor 7 is bonded tothe aluminum foam just like the lid 4.

The aluminum foam is plastically deformable to an approximately equalextent in all spatial directions and, for this reason, is isotropic. Thealuminum foam is closed cell and is encapsulated for the purposes ofwater-tightness. The aluminum foam has a density of 0.7 g/cm³, athickness of 70 mm and a diameter of 585 mm when circular. The 0.2%yield point of the load distributor 7 in the form of a circular loaddistribution plate made from fine-grain construction steel is 1100 MPa.The thickness of the load distributor 7 is 20 mm.

FIG. 2 illustrates a different embodiment of the container according tothe present invention. The same components of FIG. 1 are here providedwith the same reference characters. Compared to the embodiment of FIG.1, the container according to FIG. 2 has an additional inner lead shield9 entirely lining and delaminating the chamber 5. Here, this lead shield9 is inside the container at the end wall 2 as well as also on the sidewall 3 and a lead shielding lid 10 is provided on the lid 4. In theembodiment according to FIG. 2, the lead shielding lid 10 is kept andfixed to the lid 4 by a holding plate 11 itself fixed to the lid 4 byscrews 12 that extend through the lead shield lid 10. Thus,advantageously and in the illustrated embodiment according to FIG. 2,the assembly of the lead shielding lid 10 and holding plate 11 isinterposed between the deformable plastic layer 6 of aluminum foam andthe lid 4.

In the embodiment according to FIG. 3, the holding plate 11 is omitted.Instead, the lead shielding lid 10 is fixed to the lid 4 by the loaddistributor 7 or by the load distribution plate. The screws 12 in FIG. 3reach here from the load distributor 7 or the load distribution plate 7through the force-distribution layer 5 and the lead shield lid 10 intothe lid 4.

We claim:
 1. A container for radioactive inventory comprising: an endwall, a side wall, and a container lid that form a closed chamber forthe radioactive inventory; a plastically deformable layer between thecontainer lid and the inventory and of such composition that, in theevent of an impact of the inventory against the container lid, at leasta majority of the impact forces are uniformly distributable over atleast a majority of a surface of the plastically deformable layer. 2.The container defined in claim 1, wherein the plastically deformablelayer has a specific energy absorption of 10 to 50 J/cm³ in a verticaldirection.
 3. The container defined in claim 1, wherein the plasticallydeformable layer is isotropically deformable.
 4. The container definedin claim 1, wherein the plastically deformable layer is a metal foam. 5.The container defined in claim 4, wherein the plastically deformablelayer contains aluminum.
 6. The container defined in claim 1, whereinthe plastically deformable layer is encapsulated or coated.
 7. Thecontainer defined in claim 1, wherein the plastically deformable layerhas a thickness of 30 to 200 mm.
 8. The container defined in claim 1,wherein the plastically deformable layer is of circular shape.
 9. Thecontainer defined in claim 1, wherein the plastically deformable layeris attached directly to an inner face of the container lid.
 10. Thecontainer defined in claim 9, wherein the plastically deformable layeris bonded unitarily to the inner face of the container lid.
 11. Thecontainer defined in claim 1, further comprising: a lead shield liningthe side wall and end wall; a shield lid lining the container lid; and aflat holding element for the shield lid interposed between theplastically deformable layer and the shield lid, the plasticallydeformable layer being attached directly to the lead shielding lid or tothe flat holding element.
 12. The container defined in claim 1, whereinedges of the plastically deformable layer at least in areas border afluid.
 13. The container defined in claim 1, further comprising: a loaddistributor between the plastically deformable layer and the inventory.14. The container defined in claim 12, wherein the load distributor is aplate.
 15. The container defined in claim 1, further comprising: areversible fastener securing the container lid to the side wall.
 16. Thecontainer defined in claim 1, wherein the side wall has a thickness of100 to 350 mm.
 17. A method of making a container for radioactiveinventory comprising the steps of: providing an end wall, a side wall,and a lid together forming a closed chamber for the radioactiveinventory; interposing between the lid and the inventory at least oneplastically deformable layer of such composition that impact forces ofat least a majority of the inventory are uniformly distributable over atleast a majority of a surface of the plastically deformable layer in theevent of the container being dropped.
 18. The method defined in claim17, wherein the plastically deformable layer is formed by in situfoaming of aluminum in the container between the container lid and theinventory such that the layer bonds unitarily to the container lid.